This invention relates to an RF, or microwave, intrusion detection system, and more particularly to such a system wherein the sensing element may follow the terrain into which it is disposed irrespective of its contours, and it is an object of the invention to provide an improved intrusion detection system of this nature.
Intrusion detection systems frequently are intended for use as perimeter surveillance systems in connection with a fence or wall that surrounds the area to be protected. The idea is that some intruder coming within a specified distance of the boundary provided by the perimeter will cause an alarm to be sounded at an appropriate central location.
Several types of equipment for this purpose are presently available. These comprise seismic or vibration sensors mounted on a fence or in a buried cable, microwave links which alarm when the beam is interrupted between a transmitter and a receiver and low frequency electric field detector schemes which sense changes in the electric field between two or more parallel wires, the electric field in the latter instance being generated at low frequency and being essentially non-radiating. A more recent development also designed for perimeter surveillance may be referred to as a guided radar. In this system a pair of slotted coaxial cables separated by five to six feet are laid around the perimeter of the area to be protected. A pulse VHF transmitter is connected to one cable and a receiver is connected to the other cable. The remote ends of the two cables are terminated in appropriate characteristic impedances. A moving object between the two cables will change the received power, and the distance to the disturbance from the transmitter and receiver can be measured with a resolution compatible with the pulse width.
While all of the above systems are being used, they also have limitations that prevent or limit their use in specific applications. The vibration sensors have false alarm rates that are too high and do not detect intrusions that vault the fence. Microwave links are line of sight devices and are not suitable for rough, uneven topography or for very narrow corridors. The electric field sensor is plagued with high maintenance costs and is susceptible to lightning and interference from power lines. The leaky cable guided radar overcomes many of the indicated limitations, but it is very expensive, has little flexibility in changing the detection zone, and requires specially designed cable to equalize the sensitivity as a function of distance from the sensor.
It is a further object of the present invention to provide a low-cost, terrain following perimeter intrusion detection system compatible with other types of sensors such as microwave links and with few of the limitations characteristic of the known systems. More particularly it is an object of the invention to provide such a system wherein the sensitivity to the presence of an intruder is the same irrespective of whether the intruder is adjacent to the transmitter or remote therefrom so long as he is at the same distance from the sensing element.
In a preferred form of the invention the receiver may be of the doppler type for improved operation.